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Description of Attached Appendix
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This invention relates generally to the field of loading ramps for ships and more specifically to a roll-on/roll-off system and process for equipment transfer between ships or a ship and quay.
Bridges, ramps and trusses have been used for millennia and constitute a mature field of engineering. New designs have evolved as new applications have arisen, larger spans have been required, it has been necessary to carry heavier weight or new materials have become available. New materials and assembly processes have led to changes in design producing larger, stronger and lighter structures. The weight of a bridge or ramp is important because it can be a significant proportion of the total weight that must be supported.
One emerging application is for a ramp spanning two platforms that may be moving about fixed points. Examples are between two ships, between a ship and a quay or between deep-sea oil rig platforms. Such ramps have been used in so-called roll-on/roll-off ships. When deployed these ramps allow a container to be driven directly between a ship and a quay. In military applications very heavy equipment such as tanks may be loaded or unloaded in this way.
As ships become larger there are many instances when they must be offloaded in deep water onto smaller ships in order to transfer their cargo to port. Military equipment may need to be offloaded where no port is available. In these situations the swell can cause substantial motion, especially in the smaller ship, making roll-on/roll-off cargo transfer impossible in all but the most benign conditions. In addition, the weight of existing ramps, which typically are 30 meters or longer, limits the length of the span. Clearly there is a need for a roll-on/roll-off cargo transfer system that can accommodate the conditions that prevail in deep water. It is also clear that such a system should be lighter in weight than ramps currently in use.
A number of designs have been proposed for ramps or bridges allowing vehicles or passengers direct access to a ship or floating platform. Most of these use one or more rigid ramps attached with hinges to accommodate some types of motion between the two ends. Hetmanski (xe2x80x9cRamp engagement devicexe2x80x9d, U.S. Pat. No. 3,735,440-1973) teaches the design of one type of hinge that allows a rigid ramp to pivot and disengage when necessary. Kummerman (xe2x80x9cMovable access ramp for vehiclesxe2x80x9d, U.S. Pat. No. 3,846,860-1974), Vulovic (xe2x80x9cLoading ramp securing systemxe2x80x9d, U.S. Pat. No. 3,971,090-1976) and Vulovic (xe2x80x9cShip loading rampxe2x80x9d, U.S. Pat. No. 4,043,288-1977) use horizontally hinged, rigid ramp sections to accommodate the difference in height between a quay and a ship""s cargo deck allowing for changes due to tides or loading. Rolling is also accommodated. In all three of these patents one or both ends of the ramps may slide. While this may allow some slight fore-and-aft, lateral, or skew motions of the ship the range of motion is extremely limited.
Mori et al (xe2x80x9cSlidable mobile bridgexe2x80x9d, U.S. Pat. No. 3,715,769-1973) teaches means to position a ramp vertically and horizontally relative to a ship. The positioning means is then disengaged from the ramp which simply rests on the ship and quay. Again, this allows only a limited range of motion between ship and quayside.
Subsequent inventors added ball-joints to some ramp sections to allow greater freedom of movement. Stress was also reduced in these designs since ball-joints transmit no moments. Serrano (xe2x80x9cFootbridge for connection between a fixed installation and an oscillating installationxe2x80x9d, U.S. Pat. No. 4,162,551-1979) describes a permanent bridge with a rotating platform at one end and a platform supported on a ball-joint at the other. Three hinged, rigid sections are used to connect these platforms. Six degrees-of-freedom are accommodated with this design, however, the two end sections must have an acute angle from horizontal in order to allow lateral movement. This angle makes it impossible for such as structure to be used for vehicles in roll-on, roll-off applications. Wipkink et al adopted a similar approach (xe2x80x9cConnecting bridge for personnel to connect two mutually movable marine structuresxe2x80x9d, U.S. Pat. No. 4,169,296-1979). Two sections were used and an additional pivot, with a vertical axis, was provided between the sections. This patent has no teaching regarding the angle of the ramp sections from the horizontal. If these sections are nearly horizontal, as would be desired for traversal by vehicles, lateral movement could not be accommodated.
Lucien (xe2x80x9cRamp apparatusxe2x80x9d, U.S. Pat. No. 4,581,784-1986) uses a single rigid ramp section with a gimbal at one end and roller at the other to accommodate relative movement. It would be difficult for vehicles to traverse the gimballed end of the ramp where rapid, extreme motions would occur.
In Rawdon et al (xe2x80x9cHinged cargo rampxe2x80x9d, U.S. Pat. No. 5,253,381-1993) two ramp sections are horizontally pivoted with an additional pivot, between the ramp sections, that is oriented in the longitudinal direction. Only a limited number of degrees of freedom can be accommodated.
Kane et al specifically address roll-on, roll-off applications (xe2x80x9cRamp junctionxe2x80x9d, U.S. Pat. No. 5,359,746-1994). A rigid ramp is fixed to a quay by a kingpin, allowing certain degrees of freedom, and to a floating platform by sliding feet. The range of motion that may be accommodated is intentionally limited by shackles.
Sekiguchi et al (xe2x80x9cShip weight cargo loading and unloading systemxe2x80x9d, U.S. Pat. No. 5,511,922-1996) deal with the problem of matching the motion at the end of a ramp to a stationary deck. Vehicles are carried on a lift table that can tilt about two axes to match movement of the ramp. Castelli et al also deal with this problem (xe2x80x9cDynamic ramp interface systemxe2x80x9d, U.S. Pat. No. 6,192,541-2001). A platform is disposed between two ramps that can accommodate rotations about two horizontal axes. The end of each ramp is provided with xe2x80x9cfingersxe2x80x9d that form the transition between each ramp and the platform. Although this is claimed to be useful in high sea states the transition between rigid ramp sections is still made over only a relatively short portion of the ramp""s length.
All of the aforementioned structures share a common feature: the ramp is composed of rigid sections and relative motion, where it is allowed, is concentrated at specific points. This is undesirable for roll-on, roll-off applications where the range of motion may be large, such as in high sea-states. Stresses, especially dynamic stresses, are extremely high at the points where the ramp is attached.
Streeter et al (xe2x80x9cMethod and apparatus for connecting a passenger boarding bridge to a movable bodyxe2x80x9d, U.S. Pat. No. 5,950,266-1999) address the problem of positioning a ramp in a passenger boarding bridge such as is widely used in air and ferry terminals. They provide a system of sensors that control the movement of the passenger bridge in order to maintain a constant attitude of a bridging ramp. While this can accommodate small amplitude motions the size and mass of the passenger bridge make it impossible to reach large amplitudes and velocities.
Three patents teach the use of a flexible ramp for connecting movable platforms. Fisher (xe2x80x9cFlexible staging platform and the likexe2x80x9d, U.S. Pat. No. 3,994,036-1976), Ryan (xe2x80x9cCombined marine ramp transfer and mooring systemxe2x80x9d, U.S. Pat. No. 4,003,473-1977) and McLain (xe2x80x9cArticulated bridgexe2x80x9d, U.S. Pat. No. 6,292,968-2001) teach the use of ramps or bridges that can flex to accommodate displacements at the ends as well as rotations about the vertical axes. Stresses at the mounting points are greatly reduced by this flexure. All of these structures are, however, designed for light loads or short spans. The extra strength that would be required for roll-on, roll-off applications would necessitate stiffening of these structures, reducing their ability to deform and transmitting very high forces to the points of attachment.
An ideal ramp structure would combine the ability of these flexible ramps to twist about the longitudinal axis with the hinged pivots taught by many other inventors. Such a structure must have a high load-carrying capacity, despite its ability to twist. In addition, it is desirable that the weight of such a ramp structure is reduced as this weight can be a considerable proportion of its load-carrying capacity.
The primary object of the invention is to allow equipment transfer in rough seas with a maximum average wave height of at least 15 feet.
Another object of the invention is to allow the loading and unloading stations to move about a point with six degrees of freedom during transfer.
A further object of the invention is to reduce the forces transmitted from the movement of one station to the other.
Yet another object of the invention is to accommodate a larger range of motion than current roll-on/roll-off ramp designs.
Still yet another object of the invention is to decrease the torsional stiffness of the transfer system below that of current roll-on/roll-off designs.
Another object of the invention is to reduce the stresses at the attachment points of the transfer system.
Another object of the invention is to carry heavier weights than existing roll-on/roll-off ramps.
A further object of the invention is to weigh less than existing roll-on/roll-off ramps.
Yet another object of the invention is to decrease the cost of roll-off/roll-on systems.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.
In accordance with a preferred embodiment of the invention, there is disclosed a machine comprising: at least two beams disposed between two moveable stations, each of said beams being fixed with respect to the first of said stations such that there is no horizontal or vertical displacement or rotation about said beams"" respective longitudinal axes; a support means for each of said at least two beams at the second of said moveable stations, said support means allowing displacement along, and rotations normal to, said beams"" respective longitudinal axes; connecting means between said at least two beams that provides constant spacing without substantially increasing torsional stiffness; a transfer shuttle that can traverse the length of said at least two beams thereby allowing roll-on/roll-off equipment transfer without the need for a continuous deck and a means for imparting motion to said transfer shuttle.
In accordance with a preferred embodiment of the invention, there is disclosed a process for equipment transfer between two moveable stations comprising: moving a shuttle to a point adjacent to one of said transfer stations; transferring a load onto said shuttle; moving said shuttle to a point adjacent to the second of said transfer stations; transferring said load from said shuttle to said second transfer station and returning said shuttle to said first transfer station.